Magnesium, aluminum and their alloys have found a variety of industrial applications. However, because of the reactivity of such light metals, and their tendency toward corrosion and environmental degradation, it is necessary to provide the exposed surfaces of these metals with an adequate corrosion-resistant and protective coating. Further, such coatings should resist abrasion so that the coatings remain intact during use, where the metal article may be subjected to repeated contact with other surfaces, particulate matter and the like. Where the appearance of articles fabricated of light metals is considered important, the protective coating applied thereto should additionally be uniform and decorative. Heat resistance is also a very desirable feature of a light metal protective coating.
In order to provide an effective and permanent protective coating on light metals, such metals have been anodized in a variety of electrolyte solutions. While anodization of aluminum, magnesium and their alloys is capable of forming a more effective coating than painting or enameling, the resulting coated metals have still not been entirely satisfactory for their intended uses. The coatings frequently lack the desired degree of hardness, smoothness, durability, adherence, heat resistance, corrosion resistance, and/or imperviousness required to meet the most demanding needs of industry. Additionally, many of the light metal anodization processes developed to date have serious shortcomings which hinder their industrial practicality. Some processes, for example, require the use of high voltages, long anodization times and/or volatile, hazardous substances.
One method of magnesium anodization is described in U.S. Pat. No. 5,792,335. This method involves the use of an electrolytic solution containing ammonia. A magnesium-based material is placed as an anode in the electrolytic solution, together with a cathode, and a current is passed between the anode and the cathode through the electrolytic solution so that a coating is formed on the magnesium-based material.
Such a process is somewhat difficult to implement on a commercial scale due to the requirement that ammonia be present (preferably, at a concentration of 5-7% w/v) in the electrolytic solution. Due to the volatile and corrosive character of ammonia, the equipment used in such a process must be carefully designed and operated so as to prevent the escape of ammonia from the electrolytic bath into the workplace. This requirement will substantially increase the cost of implementing and operating an anodization process of this type.
The aforementioned patent also teaches that the electrolytic solution may contain a phosphate compound, but cautions that the use of phosphate concentration greater than 0.2M should be avoided because of surface appearance problems. The preferred phosphate compound concentration is from 0.05 to 0.08M. The patent further teaches that the process should be conducted using relatively high voltage direct current (i.e., 170 to 350 volts) and that spark formation during operation of the process should be avoided in order to minimize the current drawn and to prevent the bath temperature from increasing to an unfavorable extent.
While the aforedescribed process is capable of producing good quality, corrosion-resistant coatings on magnesium materials, the rate of coating formation (typically, 1-3 microns per minute) is lower than would be desirable.
Thus, there is still considerable need to develop alternative anodization processes for light metals which do not have any of the aforementioned shortcomings and yet still furnish corrosion-, heat- and abrasion-resistant protective coatings of high quality.